A major goal of animal vocal communication studies has been to discover the referents of signals. Such information is critical to an understanding of the function and evolution of communication systems in different species and is also important to advancing knowledge about nonhuman primate cognitive and conceptual abilities. Studies have indicated signals can represent objects and events in the external world (i.e., have the property of external reference) such findings raise the possibility that animal communication might share the denotative context independence of human language symbols. A complicating issue in efforts to categorize the primate world reflected in communication is that researchers have approached the problem as though the animals classify items with a logic system in which there are only two truth values, 1 or 0 (true and false). Although studies have established associations between different vocalizations and referents using this simple dichotomy, it is likely that such bivalent logic is not the actual basis for the categorization of many things communicated about by monkeys. We examined rhesus macaque scream vocalizations provoked by higher-ranking aggressors in two contexts encounters in which the rank-difference between opponents was either great (LRD = large rank difference) or small (SRD = small rank difference). These vocalizations play an important role in eliciting support from the caller's allies in the group (usually matrilineal kin). 'Noisy' screams, for example, are most likely to be given during encounters against higher-ranking opponents that involved contact aggression, and 'tonal' or 'undulated' screams are most likely for non-contact aggression from such opponents. It was predicted that scream bouts directed to SRD opponents would be more likely to comprise calls from scream classes other than the expected type, and that scream bouts from LRD encounters would be more in accord with the expected class. Results s upport this prediction and suggest that a multi-valued or fuzzy logic system, one with more than two truth values, might provide a more realistic way to conceptualize the categorization of certain referents of monkey vocalizations. Better understanding of the ways in which nonhuman primates categorize their worlds will enable more comprehensive and complete studies of vocal and behavioral development.